When the condition of a river or the like is monitored, it has been frequently performed to dispose a monitor camera or a sensor at each place under surveillance, collect data from these places by a monitoring center and monitor these data to perform the surveillance of the river or the like, for example. In this case, it is general that the collected data are withdrawn as electrical signals and converted to image signals or the like.
However, there has been a problem that it is impossible to continue the surveillance when an electric cable is cut or power supply is cut off under such a bad weather as a typhoon or the like.